Combined genetic and molecular techniques are applied to the analysis of a novel metabolic cross-connection between sphingolipid and nitrogenous phospholipid synthesis. Genetic mutants of the yeast Saccharomyces cerevisiae have been identified that exaggerate this biochemical connection, making degraded sphingolipid precursors serve as growth-supporting phosphatidylethanolamine precursors. Other mutants, that perhaps cannot degrade sphingolipids, suggest a previously unsuspected vital role for phosphatidylethanolamine in normal eukaryotic cells, perhaps provided by the connection between the two nitrogenous lipid biosynthetic pathways. Proposed studies utilize the cloned normal genes that correspond to the mutants to examine normal regulation of sphingolipid synthesis and coordination of this and other lipid biosynthetic pathways. The need for phosphatidylethanolamine to maintain non-chromosomal plasmids and mitochondrial genomes is under investigation in normal and mutant cultures. The greater physiological significance of phosphatidylethanolamine and the diverse mechanisms for making it are under investigation in mutants with anomalous ethanolamine requirements. These studies probe the genetic regulation of nitrogenous lipid synthesis and the need for these specific lipids in a model eukaryote. This work will contribute to a basic understanding of lipid metabolism, and may indirectly contribute to effective management of human lipid disorders that lead to neurological and cardiac abnormality.